This invention relates to a method and means for testing high impedance, ion selective electrodes while they are being used for measurement so as to determine whether or not the ion selective membrane of the electrode is damaged. It is useful to make this determination while the electrodes are being used to determine the ion activity or some property of activity such as pH, concentration, etc. of the solution being measured. In the past, it has been necessary for those who desire to make measurements of pH or sodium ion concentration, for example, to occasionally examine the electrodes when those electrodes include a high resistance membrane such as a glass membrane so as to determine whether or not the membrane was damaged as by being cracked or broken. This has been necessary since a cracked glass electrode, for example, usually provides a steady potential and thus a steady reading which gives no indication to the operator that the measurement being made is incorrect and that the electrode is defective.
It is, of course, desirable that electrode systems should not require frequent attention or examination, and that the user should be able to depend on the measurements made with them, particularly in industrial applications where they are frequently applied to the continuous measurement of process fluids. It is therefore an object of this invention to provide a method for testing of the electrodes to determine if the membrane is damaged so as to make unnecessary a visual examination of the electrode. Since the membrane of ion selective electrode systems typically have an impedance which is greater by a factor of ten than the combined impedance of the associated reference electrode and the solution being measured, it is advantageous to detect a damaged membrane by measuring the impedance of the electrode system and identifying a drop in resistance below the expected value as an indication of damage. It has been established in the prior art that the bulk resistance of a membrane such as a glass membrane can be measured by passing a unidirectional test current through the glass for a short period and measuring the voltage drop resulting from that current to thereby determine if the resistance of the glass is as high as it should be. Such measurements, however, have been only used when the ion selective electrode was not being used for continuous measurement. Such a method would not be satisfactory for electrodes applied to continuous measurements for the unidirectional current would cause polarization of the electrode, thus producing erroneous measurements. The present invention overcomes that problem.